Dextran is a highly water‐soluble polysaccharide of bacterial origin, produced by Lactobacillus, Leuconostoc and Streptococcus species. It is composed predominantly of α‐1,6‐linked glucopyranose units with a low degree of 1,3‐branching (Figure 5). Native dextran is characterized by high Mw and polydispersity, both of which can be tailored by controlled hydrolysis and subsequent fractionation. Low Mw dextran in particular already has a long history of clinical applications in humans, e.g. as plasma volume expander, to enhance peripheral blood flow or as rheological excipient in artificial tears. The combined advantages of its hydrophilic character, biocompatibility, biodegradability and ease of chemical derivatization all relate to its suitability as drug delivery biopolymer [60].
Figure 5: Chemical structure of dextran
Dextran is used to form biodegradable hydrogels, which are three‐dimensional, hydrophilic matrices that are able to retain large amounts of water or biological fluid. Dextran hydrogel NPs (dextran nanogels, dex‐NGs) can be prepared by an inverse emulsion photopolymerization of dextran hydroxyethyl methacrylate (dex‐HEMA) in mineral oil. To incorporate cationic charges in the nanoscopic hydrogel network, cationic methacrylate monomers were copolymerized with dex‐
HEMA. An important feature of these hydrogel particles is their biodegradability under physiological conditions, owing to the hydrolysable carbonate ester linking the HEMA moieties to the dextran backbone. Preformed cationic dex‐NGs could encapsulate anionic siRNA based on electrostatic interaction with a maximal loading exceeding 50 pmol of siRNA per μg of lyophilized dex‐NGs [61]. Due to this ‘post‐loading’ mechanism, the siRNA integrity is not affected by the emulsion polymerization process [62]. Different types of dex‐NGs have been described in the literature with the aim to enhance intracellular nucleic acid delivery [63, 64]. However, reports show that dex‐NGs likely possess insufficient circulation times to enable an adequate extravasation and accumulation in the tumor tissue in vivo [64]. In an attempt to tackle this problem, Naeye et al.
[65] investigated different ways of PEGylating dex‐NGs and concluded that successful PEGylation of cationic dextran nanogels was only obtained by covalent attachment of NHS‐PEG to the reactive amine groups of the nanogels. Surface decoration by physical adsorption of PGA‐PEG resulted in successful coating but a complete release of encapsulated siRNA. Another issue revealed by
confocal fluorescence microscopy upon using dex‐NGs is that a substantial fraction of the NGs accumulates in the endosomes after being internalized by the cells [66]. This prompted researchers to investigate the use of photochemical internalization (PCI) enhanced endosomal escape into the cytoplasmic compartment. PCI is an established endosomolytic method that involves the use of amphiphilic photosensitizers (PS) that accumulate in the membranes of endocytic vesicles, upon illumination with a specific light source, excitation of the PS compound induces the formation of reactive oxygen species locally, to selectively disrupt the endosomal membranes, releasing the entrapped NPs into the cytosol. It was found that application of PCI, even several days post‐
transfection, was able to significantly improve and lengthen the gene knockdown obtained with siRNA‐dex‐NGs, indicating that PCI is able to liberate a fraction of the siRNA or siRNA‐dex‐NGs that remain trapped in intracellular organelles. Applying vesicular compartments as drug depots may thus be regarded as a potential strategy to prolong the therapeutic response when endosomal escape is effect‐limiting [66, 67].
Moreover, hydrophobic modifications of dextran were reported. Acetal groups were chemically added to dextran to form acetal‐modified dextran (Ac‐DEX) where the addition of this hydrophobic moiety make the modified Ac‐DEX soluble in organic solvents but insoluble in water, allowing the production of Ac‐DEX‐based micro‐ and nanoparticles for protein antigen encapsulation via standard emulsification methods [68]. These acid‐degradable particles were further optimized for the intracellular delivery of siRNA by incorporating small amounts of spermine. Spermine is a tetravalent organic amine that is present in mammalian cells at millimolar concentrations and is considered non‐cytotoxic [39]. Authors argue that the cationic nature of spermine enables a better complexation of the negatively charged siRNA and improved interaction with the target cell membrane. Following endocytic uptake, acid‐catalyzed hydrolysis of the particles will occur in the endolysosomal compartments releasing the siRNA. The siRNA is thought to reach the cell cytoplasm as a result of endosomal burst by virtue of an amine‐induced proton sponge effect. This is further potentiated by intraluminal osmotic pressure build‐up through endosomal accumulation of spermine‐Ac‐DEX degradation products [69].
Alternatively, the negatively charged dextran sulfate (DS) can be used to prepare nanosized polyelectrolyte complexes (PECs) together with cationic biopolymers, such as chitosan. PECs result from direct interaction of oppositely charged polyelectrolytes in solution and within PEC NPs, polycations are the driving force for complexation of negatively charged nucleic acids. PEC structure
and stability are primarily influenced by the polymer properties (Mw, flexibility, charge density) and the chemical environment (pH, ionic strength, temperature) [14]. The rationale behind the use of (polysaccharide) polyanions as an additive in PEC drug formulations is to create more stable nanocomplexes, to minimize polycation‐induced toxicity and/or to stimulate cellular interactions via carbohydrate‐binding receptors [70].
To the best of our knowledge, very little was yet published on the use of dextran‐based siRNA delivery in vivo. Cho et al. [70] reported on PECs composed of DS and the positively charged poly‐L‐
arginine (PLR) polypeptide to encapsulate siRNA targeting the epidermal growth factor receptor (EGFR). The incorporation of polyanions, such as dextran sulfate or hyaluronic acid, has been shown to result in more compact siRNA PECs compared to PECs formed between positive polycations and anionic siRNA molecules only [71]. Significant in vivo tumor growth inhibition in head and neck cancer cells was demonstrated using the optimized PEC formulation after intratumoral injection in a xenograft mouse model [70].